The rabit reticulocyte can be fractionated into four primary components: the plasma membrane; mitochondria; polysomes; and cytosol. Lysosomes comprise a relatively minor fraction of the cell. The cytosol can be fractionated by gel filtration chromatography and the following iron components obtained: ferritin; transferrin (it is unknown if this is "intracellular transferrin" or simply transferrin released from the plasma membrane as a result of cell lysis); hemoglobin; an approximately 18,000 dalton iron binding component; and an approximately 6,000 dalton iron: binding component. The goal of this research is to examine the pathways in which iron is exchanged between the various subcellular and cytosol constituents listed above. It is possible to isolate all of these constituents with specifically labeled 59Fe. Through incubation procedures we will examine the pathways of heme and non-heme iron exchange. The utilization of metabolic inhibitors, in particular isonicotinic acid hydrazide, will allow us to interrupt the flow of iron at various points. Analysis of inhibition of the system with regard to depletion and enhancement of the iron binding components will help to establish the unknown iron pathways. The ability of the plasma membrane to store and exchange iron will be examined in detail. The presence of ferrochelotase activity in mitochondria will be reexamined. Iron mobilizaton from ferritin by cytosol will be tested. The isolation and characterization of the 6,000 dalton iron binding agent will be pursued as a primary goal.